Tritoon CG question

Discussion in 'Boat Design' started by Nibbler1, Apr 12, 2012.

  1. Nibbler1
    Joined: Apr 2012
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    Location: Alabama

    Nibbler1 New Member

    I'm having a 30' x 8.5' tri-toon built with 26" toons for a light commercial application. The center toon extrudes a couple of feet beyond the rear of the deck to mount the 60 Bigfoot.

    My question is;

    Is there a ratio of front to back weight distribution desired?

    In other words if my gross is 4000 lbs., how much weight should be to the rear of center-line for appropiate bow lift when traveling?

    Thank you in advance,

    Nibbler1
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Welcome Nibbler,

    CG location is a moving target. Can you tell us a bit about the application, the hoped for performance, and the conditions you plan to operate in? What does the builder suggest?
    Its a pretty normal sort of size and weight, so it shouldn't be difficult to locate a sufficiently similar vessel for comparison. The main issue is whether or not you can expect to get any meaningful lift from the angle of attack. The second issue is whether the reduction in wetted surface area with the bow pitched up will reduce friction by more than the wave resistance is increased. I think you will need to experiment a little bit but that you won't want to be bow high by very much underway at 4000# and 75% throttle. Unloaded, she might benefit from some bow high trim and travel a good bit quicker.

    My initial guess is to set the motor trimmed about 5 degrees past horizontal for the loaded case and have your payload at or just ahead of midship, assuming that the unloaded boat will have a bit of aft bias built in and run light with a flat prop if properly executed.

    Are all three toons in a line, or is the middle one lower? What is the actual length of each pontoon?
     
  3. Nibbler1
    Joined: Apr 2012
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    Location: Alabama

    Nibbler1 New Member

    Thanks for the reply PhilSweet,
    My application is a floating kitchen.
    I can move appliances around to balance any way I need to.
    I was curious as to any general rules regarding rearward weight differential, if any.
    Fully loaded with persons, food and fuel I expect around 5300 lbs.
    My builder says I'm good up to 10,000 half submerged with this tri-toon config.
    As far as speed, 10-12 knots will be all I would like to see.
     

  4. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    The reasoning behind shifting the weight aft goes something like this-

    As speed increases, the skin friction goes up dramatically. Something close to the square of the speed. If you are light enough and going fast enough, you can reduce the total drag by pitching up and planing out on a much smaller surface area than if you didn't pitch up.

    In order for the boat to balance when on a plane, the weight center has to be moved farther aft. I don't think you are going to be light enough or fast enough to make this work for you. A Lot also depends on hull shape and pontoons aren't very good at this unless they are running very light.

    Besides, don't you want to just putt-putt down the canals like the ice cream man selling teriyaki hot wings to the folks hanging out on their dock?:D

    I'm an RVer and it just drives me nuts if my frying pan isn't perfectly level. Are you going to gimbal any of your gear?

    The boat's pitch can also be affected by the throttle setting of the engine.
    If the prop push vector is below the center of resistance of the hull, it will tend to nose up as you add throttle. If you trim the outboard down and the prop is kicking the stern up, the opposite is true. So what you want to do is find the trim angle of the outboard that gives you a happy hull response to throttle input (which I guess would be none at all for a kitchen), and then shift weight to obtain best performance at that outboard trim. You might expect to change the outboard trim if it got choppy. Hopefully, your thread won't die here. If nothing else, visit our cooking thread down in the "all things boating" subforum.
     
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